Thermostatic expansion valves are often used for metering liquid refrigerants injected into an evaporator. The expansion valves are controlled by a temperature sensor containing a vapor forming liquid, the temperature sensor being mounted at the discharge of the evaporator. The control is such that the vapor pressure of the liquid contained in the temperature sensor acts on the head of the expansion valve, the desired superheating at the outlet of the evaporator being set by means of a superheating spring.
These thermostatic expansion valves have a number of drawbacks. Since the valves must be matched to the refrigerant being used, the manufacturer is forced to supply a corresponding plurality of functionally equivalent units, or, alternatively, units with exchangeable jets or nozzles of different sizes. Furthermore, the time constant of such valves, which depends on the size of the diameter of the membrane or corrugated tubing, the spring temper of the superheating spring, the selected size of the jet and the refrigerant, can not be changed once the valve has been manufactured.